Summary African-American (AA) women are disproportionately burdened by obesity. Our published data have led to the provocative hypothesis that this disparity has physiologic underpinnings. Specifically, AA women are characterized by a high acute insulin secretory response to a glucose challenge, as well as reduced hepatic insulin extraction, which together lead to postprandial circulating insulin concentrations that can be several fold higher than those observed in Caucasian (European-American, EA) women. The lipogenic actions of insulin could favor partitioning of energy to adipose tissue at the expense of ATP (energy) production. In fact, AA women have lower energy requirements than EA women, and are more energetically efficient. High insulin secretion, however, is only one factor that determines insulin action; another major factor is insulin sensitivity. Our data have shown that over time, weight (fat) gain in obesity-prone AA women is higher in those who are more insulin sensitive, whereas ?obesity-resistant? (constitutionally lean) AA women have relatively lower insulin sensitivity. Our data also suggest that the lipogenic actions of insulin in AA women are exacerbated by a high-glycemic diet, which promotes insulin secretion. Finally, our data have shown that over time weight (fat) gain in AA women is predicted by the combination of insulin sensitivity and dietary glycemic load. Taken together, these observations suggest that many AA women are predisposed to obesity by their endocrine make-up, and that this predisposition is exacerbated by a high-glycemic diet. A natural corollary of this hypothesis is that for implementation of intentional weight loss, a low- (vs high-) glycemic diet will promote greater loss of body fat, and will enable successful weight loss maintenance by increasing energy expenditure. This corollary is supported by our preliminary data that indicate a 50% greater loss of fat with low- vs high-glycemic diet under controlled conditions in AA women. The project proposed herein will test these hypotheses through a randomized clinical weight loss trial of high- vs low-glycemic diets in obese AA women. The study has both an efficacy phase (controlled feeding during weight loss) to probe physiologic mechanisms (energy expenditure, metabolic efficiency), and an effectiveness phase (6-month free-living follow-up) to test the hypothesis that the low-glycemic diet will be more effective at promoting weight loss maintenance due in part to improved adherence and quality of life, and ease of implementing the diet. This study is innovative in that it proposes to test a specific physiologic mechanism that may underlie propensity to obesity; it includes both efficacy and effectiveness outcomes; it involves both quantitative and qualitative methodology; and it tackles optimization of both weight loss and weight loss maintenance. This study is intended to provide the evidence base necessary to improve the clinical care approach to weight loss.